For example, Patent Document 1 shows an engine performing compression ignition combustion of an air-fuel mixture in a cylinder, when the engine operates in a predetermined low-speed partial-load operation range.
Patent Document 2 shows an engine performing compression ignition combustion when the engine operates in a predetermined or lower switching load range, while performing combustion by forcibly igniting using a spark plug an air-fuel mixture in a cylinder, when the operation range of the engine falls with a load operation range higher than the switching load range. In performing compression ignition combustion, this engine performs what is called “two-time opening” of the exhaust valve of opening an exhaust valve again in an intake stroke to introduce into the cylinder, part of exhaust gas that has been discharged to an exhaust side. The introduction of internal EGR gas by two-time opening of the exhaust valve increases the compression end temperature in the cylinder 18, and is advantageous in improving the ignitionability in the compression ignition and the combustion stability.
Patent Document 3 shows an engine switchable between compression ignition combustion and spark ignition combustion in accordance with the operation of the engine. Patent Document 3 also teaches introducing part of the exhaust gas into the cylinder via the EGR passage in transition from the compression ignition combustion to the spark ignition combustion, and enriching the air-fuel ratio of the mixture to reduce knocking.